The Solar Pond

A different approach to Solar thermal electricity generation is the solar pond which uses a large salty lake as a kind of flat plate collector. If the lake has the right gradient of salt concentration ( salty water at the bottom and fresh water at the top) and the water is clear enough solar energy is absorbed on the bottom of the pond.

The hot, salty water cannot rise, because it is heavier than the fresh water at the top. The upper layers of the water act as an insulating blanket and the temperature at the bottom of the pond can reach 90 degrees C. This is a high enough temperature to run an organic rankin cycle (ORC) engine or Stirling engine.

However the thermodynamic limitations of the relatively low temperatures mean low solar to electricity conversion efficiencies, typically less than 2%. Even with this low capability systems of 50 mwatts electrical output, fed from a lake of 20 hectares. The largest operating solar pond for electricity generation was the Beit HaArava pond built in Israel and operated up until 1988. It had an area of 210,000 m² and gave an electrical output of 5 MW.

Beit HaArava solar pond Israel

The first solar pond in India (6000 sq. metres) was built at Bhuj. The project was sanctioned under the National Solar Pond Programme by the Ministry of Non-conventional Energy Sources in 1987 and completed in 1993 after a sustained collaborative effort by TERI, the Gujarat Energy Development Agency, and the GDDC (Gujarat Dairy Development Corporation Ltd). The solar pond successfully demonstrated the expediency of the technology by supplying 80,000 litres of hot water daily to the plant. The Energy and Resources Institute provided all technical inputs and took up the complete execution of research, development, and demonstration. TERI operated and maintained this facility until 1996 before handing it over to the GDDC. The solar pond functioned effortlessly till the year 2000 when severe financial losses crippled GDDC. Subsequently, the Bhuj earthquake left the Kutch Dairy non-functional. Learn more about the Bhuj India Solar Pond.

The 0.8 acre solar pond powering 20% of Bruce Foods Corporation’s operations El Paso, Texas is the second largest in the U.S. It is also the first ever salt-gradient solar pond in the U.S.

A natural example of these effects in a saline water body is Solar Lake, Sinai, Israel.

Efficiency

The energy obtained is in the form of low-grade heat of 70 to 80 °C compared to an assumed 20 °C ambient temperature. According to the second law of thermodynamics (see Carnot-cycle), the maximum theoretical efficiency of a solar concentrator system with molten salt is: 1-(273+20)/(273+80)=17%. By comparison, a power plant’s heat engine delivering high-grade heat at 800 °C would have a maximum theoretical limit of 73% for converting heat into useful work (and thus would be forced to divest as little as 27% in waste heat to the cold temperature reservoir at 20 °C). The low efficiency of solar ponds is usually justified with the argument that the ‘collector’, being just a plastic-lined pond, might potentially result in a large-scale system that is of lower overall levelised energy cost than a solar concentrating system.

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Advantages:The large thermal mass of the system acts as a heat store, and electricity generation can proceed 24 hours a day.The best use of solar ponds may be to generate heat for desalinization plants, creating enough fresh water to maintain themselves and provide a supply of drinking water.

Disadvantages:Large amounts of fresh water are required to maintain the salt gradient. Large bodies of water in desert areas are few in number so locations are difficult to find.Solar Ponds are not viable at higher altitudes, since the solar collection surface area (bottom of the pond) needs to be flat not tilted.